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Old 30th August 2006, 08:39 AM   #1
panos29 is offline panos29  Greece
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Default CCSed LTP validity?

Is this a valid topology for a splitter-driver arrangement or not?
I mean its very straghtforward and seems no one is using it. Is there a something wrong with that, because I plan on building it for testing purposes. Any opinions will be appreciated.
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Old 30th August 2006, 09:14 AM   #2
EC8010 is offline EC8010  United Kingdom
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It can be made to work, but you have to be able to adjust the current setting of the CCS in the tail very finely. Essentially, the two CCSs up top are fighting the CCS down the bottom. It works in simulation, but in the real world it's finicky. It's only really worth using if you don't have enough HT to be able to afford the voltage drop caused by resistive anode loads.
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Old 30th August 2006, 09:21 AM   #3
panos29 is offline panos29  Greece
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I was thinking of the fact that the top and lower current sources are going to fight eachother and was wondering how can I adjust them.

On the other hand, I do have as much voltage as needed, but I thought the active loads will be able to lower distortion at very low levels.

I have to add that this circuit is built in order to drive a 5998A PP final stage.

Do the calculated part values seem o.k?
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Old 30th August 2006, 09:44 AM   #4
EC8010 is offline EC8010  United Kingdom
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I suggest you replace the 62R resistor with a 100R fixed resistor in parallel with the series combination of 120R fixed and 50R variable. That will swing the total resistance from 54.5 to 63 Ohms and should allow you to set your anode voltages.
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Old 30th August 2006, 10:03 AM   #5
SY is offline SY  United States
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EC, I've seen something like that in a popular text. Does anything have to be done for temperature compensation or drift to keep the finicky circuit happy?
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Old 30th August 2006, 11:00 AM   #6
EC8010 is offline EC8010  United Kingdom
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Default On the spot

In theory, the LED drifts with temperature in the same direction as Vbe, so they tend to hold the voltage across the current programming resistance constant, holding current constant. In practice, there's quite a variation between LEDs and the LED might not be at the same temperature as the transistor. If the LED is rectangular and the transistor a TO92 package, then you could epoxy them together to assist in temperature tracking, but the fact remains that there's a significant thermal resistance between the transistor die and the outside surface of the package and the LED die and its outside surface, and that will always cause errors.

Positioning: Obviously, it is not a good idea to put the circuit near localised sources of heat like transistor heatsinks or where the temperature can change rapidly. It's a good idea to put the tail CCS and the anode CCSs reasonably near to one another so that they are all at the same temperature. That way, the drift that wasn't cancelled between the LED and Vbe may be cancelled between the anode and cathode circuits to keep anode voltage approximately constant (the absolute value of current isn't important, but the anode voltages are).

Does it matter?: If the circuit has 20V across the anode CCSs but only swings 10Vpk-pk of signal, then a few volts of drift won't matter. It's all down to how closely you approach the limits. My experience is that it's more stable than you'd expect.

Test it: Wave a hairdryer at it whilst monitoring anode voltages with full expected signal applied.
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Old 30th August 2006, 11:26 AM   #7
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The dual CCS on the plates can be greatly simplified by using a current mirror (they will auto-balance then too) and adjusting the overall current solely with the tail CCS.
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Old 30th August 2006, 01:03 PM   #8
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The question about dueling current sources (plates versus cathode CCSs) in a diff amp is an interesting conundrum that I’ve faced in the past. It is a potential problem for both DC stability and for AC signal performance.

I think of it this way: If we apply a signal only to one grid, then the same triode’s cathode must drive the other cathode, as well as the cathode CCS. We want the signal current to feed only into the other cathode, and not into the cathode CCS. This requirement ensures that the diff amp acts like a true diff amp – dual but opposite outputs, with low common-mode response. This requirement means that the resistance looking into the other cathode must be much lower than the resistance of the cathode CCS. The resistance looking into the “other” cathode will be (Rx+rp)/(mu+1), where Rx is the resistance of the “other” plate’s CCS. This cathode resistance must be much less than Ry, the resistance of cathode CCS. CCSs can have rather indeterminate values of resistance. Or, said another way, the values are determined by rather sloppy, and non-linear, active component parameters, such as hfe and 1/hoe, if we are forced at gunpoint to use BJTs here. Usually the resistance value is so high that it is easily swamped by some other parallel resistance, such as a plate resistance, but that assumption cannot necessarily be made here. Even if Rx and Ry were predictable and exactly the same, the ratio of Ry to (Rx+rp)/(mu+1) would then only be about mu, hardly enough for ideal diff amp behavior. So you almost want the cathode CCS to be a “better” CCS than the plate CCS, and that stray thought should indicate a fundamental problem.

I have tried common-mode feedback circuit in the past to address these issues. If you tie a large value resistor to each plate and connect the other ends together, the voltage at the joined ends will be Vplate1 – Vplate2, which is the common-mode output voltage. Differential (desired signal) voltages are ignored. The catch is that these resistors must themselves have much larger values than the plate CCS resistance to avoid degrading the high plate load resistance, and that’s tough. In tube designs where there is already a buffer on each plate, such as a differential mu stage/SRPP or a diff amp followed by a pair of CFs, the low impedance outputs can safely feed the common-mode summing resistors without degrading the plate CCS effect. If we sample this summed voltage and then apply it (in proper polarity and with adequate gain) to a voltage-controlled CCS in the diff amp’s cathode, then that CCS is forced to react in a way that ensures that there is no common-mode signal present in the output plates, or said another way, that the output voltages are equal and opposite. Back to true diff amp behavior.

If this common-mode-only feedback loop is low-pass filtered at a very low frequency, only DC stability is addressed, although usefully. If the loop runs wide open across the audio range, then the AC signal balance across the plates is improved. But now we’ve added more active parts influencing the audio signal, although that influence is at a second order (reduced by the native CM rejection).

In my trials, I used triode-based CCSs at the plates and a pentode-based CCS in the cathode. As I recall, I buffered (with a CF) and level-shifted (with zeners and yet another CCS in the CF cathode circuit) the sample common-mode output voltage and applied it to the control grid of the pentode in the CCS. I recall that it did function, but the added complexity had exceeded its net value for that project. Also, as all the node resistances go to astronomically high values, stray capacitances now have a open playing field, especially those ugly non-linear capacitances that you get for free if you use (at gunpoint) solid-state CCS components.

Still, for the adventurous tube designer, the common-mode feedback concept might find a useful home. I’ve seen similar circuits in entirely solid-state designs, so I know that the concept has a history.

In the end, for my “simple” diff amp project, I reverted to choosing “good old” plate RESISTORS with large, but tractable, values (RL >> rp) and made a decent cathode CCS. With this standard approach, you can easily design for Ry >> (RL+rp)/(mu+1), as required for good diff amp behavior.
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Old 30th August 2006, 01:53 PM   #9
ilimzn is offline ilimzn  Croatia
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Idealizing things hee, if you had two equal CCSs in the plate circuits of a cathode coupled amp, the 'tail' current through a suitably dimensioned resistor would be constant, as the plate current has nowhere else to go. It would be interesting to see what this sort of circuit would do
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Old 30th August 2006, 02:15 PM   #10
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As a diff amp, not very well, I'm afraid.
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